The present invention relates to sensors for use in detecting gaseous components and more particularly to ceramic sensors for use in analyzing combustion emission components.
A variety of sensors have been developed for detecting different gaseous combustion emission components. Examples of the different gaseous components which these sensors can detect include, but are not limited to oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), and nitrogen oxides (NOx). These sensors can be used in a variety of devices including, for example, automotive engines, diesel engines, gas turbine engines, jet engines, power plants, furnaces, and barbecues. Many of these gaseous components are hazardous.
Information derived from these sensors can be used for a variety of purposes. Data from the sensors can be used for feedback control of different aspects of a device which is producing a gaseous emission. Alternatively, these sensors can simply be used to monitor the content of the emission. For example, these sensors can be used as a component of an on-board, OEM emissions control system for an automotive engine or as an off-board emissions measuring device used for inspection and maintenance, for example as a tool for an automotive mechanic.
A need exists for sensors which can detect a wide array of gaseous components. For example, a need exists for the sensor which can determine the concentrations of oxygen, carbon monoxide, carbon dioxide, hydrocarbons, and nitrogen oxides in a sample. The sensors should have a high signal-to-noise ratio and thus be able to accurately determine the concentrations of various components of a gaseous sample. The sensors should be simple, reliable, and inexpensive to manufacture. These and other objectives are provided by the sensors, devices, and methods of the present invention.
The present invention relates to a modified universal exhaust gas oxygen sensor, referred to herein as a CEGA sensor, which can be used to measure the concentration of a variety of components of a gaseous emission including CO, CO2, O2, H2, and H2O. The CEGA sensor employs at least one additional electrode on a ceramic substrate which possess a different catalytic activity relative to the electrodes that are normally found on a UEGO sensor. The ceramic substrate may be made of any suitable ceramic and is preferably made of zirconia.
The difference in catalytic activity between the additional electrode(s) and the electrodes native to the UEGO sensor create an oxygen gradient which enables a measure of combustion completeness to be calculated. Combustion completeness is a parameter quantifying the degree to which the gaseous emissions of combustion are in chemical equilibrium. In combination with an air/fuel ratio measured by the sensor, the concentrations of different components in the emission can be calculated.
A method is also provided for measuring concentrations of components of a gaseous emission by measuring an air/fuel ratio using a ceramic sensor, measuring combustion completeness using the ceramic sensor and determining concentrations of components of a gaseous emission based on the measured air/fuel ratio and measured combustion completeness. The CEGA sensor of the present invention enable these functions of the method to be performed by a single sensor.
In one regard, the CEGA sensor is an improved universal exhaust gas oxygen sensor (UEGO) for measuring properties of a gaseous emission which includes at least one oxygen pumping cell and a sensing cell in contact with a detection cavity, the sensing cell including a ceramic in gas communication inside the detection cavity, a first electrode in contact with ceramic positioned inside the detection cavity, and a second electrode in contact with the other side of the ceramic, a first voltage potential externally applied between the first and second electrodes for pumping oxygen across the ceramic into and out of the detection cavity, the first voltage potential controlled by a second voltage potential formed across a third and fourth electrode of the sensing cell, an air/fuel ratio measurement of the gaseous emission being obtainable from the current passing between the first and second electrodes, the improvement comprising the addition of a fifth electrode which has a different catalytic activity than the first electrode positioned inside the detection cavity in contact with the pumping cell ceramic, a third voltage potential externally applied between the first electrode and either the second electrode or a sixth electrode located on the same side of the pumping cell ceramic as the second electrode, the third voltage potential controlled by a fourth voltage potential formed between the first and fifth electrodes, a measure of combustion completeness being obtainable from the current passing between the first and the sixth electrodes.
In one particular embodiment of a CEGA sensor, the sensor includes
a detection cavity;
a diffusion passage across which the gaseous emission enters the detection cavity; an oxygen pumping cell defining a portion of the detection cavity formed of a ceramic substrate and a first electrode in the detection cavity and a second electrode outside the detection cavity for pumping oxygen into and out of the detection cavity across the ceramic substrate to maintain a target oxygen level concentration in the detection cavity, an air/fuel ratio measurement of the gaseous emission being obtainable from current passing between the first and second electrodes; and
a sensing cell defining a portion of the detection cavity formed of a ceramic substrate, the sensing cell including
a third electrode within the detection cavity,
a fourth electrode outside the detection cavity, a second voltage potential being formed between the third and fourth electrodes due to a difference in oxygen concentration across the third and fourth electrodes, and
a fifth electrode in contact with the ceramic within the detection cavity which has a different catalytic activity than the first electrode, a fourth voltage potential being formed between the fifth electrode and the first electrode due to a difference in oxygen concentration across the fifth electrode and the first electrode, a measure of combustion completeness being obtainable from a current passing between the first and the sixth electrodes.
The present invention also relates to several methods, devices and systems which can be used with various types of ceramic sensors including the CEGA sensor of the present invention in order to improve their performance.
In one regard, the invention relates to a method for calibrating a ceramic sensor which, as one of its functions, determines an air/fuel ratio. This method can be used in combination with any sensor which calculates an air/fuel ratio including, but not limited to UEGO, NOx and CEGA sensors.
According to the method, a ceramic sensor is operated at a constant, known air/fuel ratio. While being operated at a constant, known air/fuel ratio, the pumping current (Ipm) of the sensor is measured. A basic relationship which correlates the air/fuel ratio to the pumping current for the family of sensors to which the specific ceramic sensor belongs is then used to calibrate the sensor by comparing the measured pumping current (Ipm) to the expected pumping current from the basic relationship for that air/fuel ratio (Ip). A transformation between the measured pumping current (Ipm) and the current that the basic relationship gives for a known air/fuel ratio is created. During subsequent sensor usage, this transformation is used to modify the measured pumping current to create a value which is used with the basic relationship to obtain an air/fuel ratio that is accurate for the specific sensor.
In one particular embodiment, the method for calibrating a ceramic sensor which, as one of its functions, determines an air/fuel ratio includes the steps of:
operating the ceramic sensor at a constant, known air/fuel ratio;
measuring a pumping current of the sensor;
comparing the measured pumping current to an expected pumping current for the constant, known air/fuel ratio; and
calibrating the sensor using a basic relationship which provides the expected pumping current for the air/fuel ratio at which the ceramic sensor was operated.
The present invention also relates to a software algorithm which can be incorporated into a system in which the sensor is used which compares Ipm versus Ip for one or more air/fuel ratios and produces a look-up table for Ipm versus air/fuel ratio which can be used during the operation of the sensor.
The present invention also relates to a semiconductor memory device which can be used in combination with or incorporated into a ceramic sensor, the memory device including logic and data for performing a variety of functions. For example, the memory device can include logic for calibrating the sensor as well as memory for calibration data for the sensor. The memory device can also include logic and memory for storing usage information regarding the sensor. The memory device can also include logic which monitors and controls the operation of the sensor. The memory device can also include logic for detecting when the sensor is being used or has been used beyond its recommended limits, e.g., temperature, time, voltage, etc. The memory device can also include a mechanism for warning the user of the improper use or overuse.
A method is also provided for correcting for temperature transients by measuring the temperature of the sensor; and correcting an output of the sensor based on the measured temperature. The system for operating the sensor can also include logic for adjusting the sensor""s output based on a determination of the sensor""s temperature.
The present invention also relates to a method for reducing noise from leakage current from the sensor""s heater by taking measurements when the heater is off or after the effects of the leakage current have reached steady-state, most preferably just prior to turning the heater on.
The present invention also relates to a method for reducing noise due to coupling between the heater wires and sensing element""s wires by taking sensor measurements before transitions in the heater""s voltage occur.
The present invention also relates to a method for reducing noise due to the use of a sensor impedance measuring method for determining a sensor""s temperature by taking measurements just before the impedance measuring event.
The present invention also relates to logic for performing any of the above methods for avoiding noise by controlling when sensor measurements are taken. The present invention also relates to logic for determining whether the heater duty cycle is low or high and for selecting the measurement times based on the duty cycle.
The present invention also relates to a method for reducing noise due to a regulated voltage-type heater in a ceramic emission sensor system by measuring the noise due to the regulated voltage-type heater at and subtracting the noise from the sensor signals in order to compensate for this source of noise.
The present invention also relates to a method for improving the accuracy of measuring oxygen-containing species in a gaseous emission in multiple cavity sensors. According to one embodiment, the method is performed by applying a gaseous emission to the sensor; measuring a pumping current in a first cavity of the sensor which has a functional relationship to an air/fuel ratio of the gaseous emission; measuring a pumping current in a second cavity of the sensor which has a functional relationship to an amount of oxygen-containing species in the gaseous emission and the air/fuel ratio of the gaseous emission; and using a combination of the measured pumping currents of the first and second cavities to measure an amount of oxygen-containing species in the gaseous emission. This method can be incorporated into the sensor by incorporating logic and data for performing the method into the sensor.
The present invention also relates to a method for field calibrating sensors using gaseous emissions. According to one embodiment, the method of field calibration is performed by applying a gaseous emission having a known amount of oxygen-containing species to the sensor; measuring a pumping current in a first cavity of the sensor which has a functional relationship to an air/fuel ratio of a model gas, measuring a pumping current in a second cavity of the sensor which has a functional relationship to the amount of oxygen-containing species in the gaseous emission and the air/fuel ratio of the gaseous emission; and using a combination of the measured pumping currents of the first and second cavities and the known amount of oxygen-containing species in the gaseous emission to calibrate the sensor. This method can be incorporated into the sensor by incorporating logic and data for performing the method into the sensor.
The present invention also relates to a method for minimizing the effect of rapid emission composition transients on the accuracy of multi-cavity exhaust sensors. According to the method, the effect of rapid emission composition transients on the accuracy of a multi-cavity exhaust sensor is minimized by measuring the sensor values; detecting for an occurrence of a rapid emission composition transient; discontinuing usage of the measured sensor values when the rapid emission composition transient is detected; detecting for a subsidence in the rapid emission composition transient; and resuming usage of the measured sensor values when subsidence of the rapid emission composition transient is detected.